Causes, extent, and consequences of an apparent host range expansion of emerald ash borer in North America
Emerald ash borer is an invasive Asian pest of ash species in North America. I recently discovered that emerald ash borer was capable of using white fringetree (Chionanthus virginicus L. (Oleaceae), a native North American species, as a larval host in the field. The full extent of the susceptibility of this species to EAB remains to be determined, as is the susceptibility of close relatives that are exposed to this beetle. Studies have revealed that Asian ashes, such as Manchurian ash, are more resistant to attack than are North American ashes. Like Manchurian ash, Chinese fringetree (Chionanthus retusus) has a native distribution that overlaps with EAB in Asia and it may prove to be more resistant to EAB than white fringetree due to its shared evolutionary history and possession of effective defense mechanisms. The degree of susceptibility of Chinese fringetrees to EAB attack relative to white fringetree has never been studied, nor has the comparative susceptibility of either host to other wood borers. I examined ornamental white and Chinese fringetrees planted in Spring Grove Cemetery and Arboretum in Cincinnati, Ohio for evidence of attack by EAB and lilac borer, a native wood borer. I also examined the performance of EAB larvae on these and related hosts in the laboratory.
Nine of the 28 white fringetrees examined possessed one or more exit holes characteristic of EAB and also had feeding galleries characteristic of EAB larvae when examined further. Most of the trees were showing some branch dieback and epicormic sprouting as a result of borer attacks. Attacked trees were distributed fairly randomly across the cemetery and had significantly larger basal stem diameters than unattacked trees. Nineteen of 28 white fringetrees exhibited evidence of lilac borer attack, and attacked trees were again larger on average than unattacked trees. Eight of nine white fringetrees with EAB attack also had lilac borer attack. None of the 11 Chinese fringetrees examined showed evidence of attack by either EAB or lilac borer. In the laboratory, EAB larvae penetrated all species initially, but their survival and performance was significantly greater in white than in Chinese fringetree. White fringetree is proving to be an acceptable host to EAB across a wide geographic range in the field, but coevolved species, like Chinese fringetree appear to be resistant. Other as yet unexamined, but closely related species in the Oleaceae are worthy of attention.